Partly implantable systems may use magnets to hold internal and external pieces in place. For example, as shown in FIG. 1, a cochlear implant 102 located under the skin 103 and embedded in bone 104 typically include a first magnet 106 placed in the center of the implant 102, and a coil 108. An external part 101 includes a second magnet 105 that is positioned over the first magnet 106 such that the external part 101 is held against the implant 102 in an optimum position. By maintaining such a position, an external coil 107 positioned inside external part 101 can, via inductive coupling, transmit a transcutaneous signal and/or power to the coil 108 of implant 102.
Upon a wearer of such a cochlear implant 102 having to undergo Magnetic Resonance Imaging (MRI) examination, interactions between the implanted magnet 106 and the applied external MRI magnetic field may, at higher field strength (i.e. above about 1 Tesla), produce two harmful effects. First, as shown in FIG. 2, the implanted magnet 202 may experience a torque that can twist the magnet 202 and the implant 201 out of position, thereby injuring the implant wearer, as shown in FIG. 2. Secondly, due to the external magnetic field, the implanted magnet may become partly demagnetized and may not be strong enough after the MRI field exposure to hold the external part in place.
Another potentially dangerous effect may occur when RF pulses emitted by the MRI unit induce voltages in the implant coil, implant circuit and/or electrode circuit. These voltages may generate unwanted stimulation, especially in implants with analog electronic circuitry. Additionally, over-voltages may be generated which could destroy the implant electronics.
Still other adverse effects can occur when a patient with a cochlear implant undergoes an MRI examination. For example, artifacts may appear in the MRI image. These artifacts are caused by the local magnetic field of the implanted magnet, which distorts the homogeneous MRI field.
Present efforts to address the above-described problems include generally forbidding a patient with the cochlear implant to undergo the high-field MRI examination. However, this may exclude the patient from certain important diagnostic measures. Alternatively, the implant can be designed to minimize certain risks related to MRI examinations. For example, to avoid risks related to the implant magnet, the implant could be designed in such a way that the magnet can be removed from the patient before MRI examination and be reinserted afterwards. This requires two surgical interventions in order to perform a MRI examination, and makes it impossible to use the implant system during the healing phase of the incision. Furthermore, the necessity to surgically remove the magnet before the MRI examination is a drawback especially in emergency cases and even could be forgotten in some cases. Other ways to minimize the risk of a torque exerted on the implant magnet is to use two identical implant magnets with opposite orientation, as described in U.S. Pat. No. 6,348,070 issued to Teissl and Hochmair, or to use a magnetically soft material (also called a “keeper”) instead of a permanent magnet inside the implant. At present, the magnetically soft material used for the keeper has been limited to solid alloys or Ferrite.
Current methodologies to reduce the risks related to induction of possibly excessive voltages in the implant coil and circuits during an MRI examination include, for example, adding a Zener diode or a similar electronic component to the electronic circuit. Other designs include the use of two implant coils with opposite direction so as to reduce the induced voltages in the implant coils, and the use of REED contacts as described in U.S. Pat. No. 6,348,070 issued to Teissl and Hochmair.